Method of making synthetic diamond wear component

ABSTRACT

A method is disclosed for making a wear component that includes providing a base surface, producing a synthetic diamond film having at least a particular equivalent strain, and applying the diamond film to the base surface. A method is also disclosed for producing synthetic diamond for use as a wear surface, by chemical vapor deposition wherein the equivalent strain of the synthetic diamond is monitored, and deposition parameters are modified when the equivalent strain of the synthetic diamond is less than a predetermined percentage.

FIELD OF THE INVENTION

This invention relates to improvements in wear components havingsynthetic diamond wear surfaces and, more particularly, to improvedsynthetic diamond for use in wear components, to methods of making suchdiamond, and to methods of determining the suitability of syntheticdiamond for wear applications.

BACKGROUND OF THE INVENTION

The use of natural diamond in wear components, such as in cutting andgrinding tools, is very old. In addition to extreme hardness, diamond'ssuperlative thermal conductivity, thermal stability, and inertness areunsurpassed for wear applications. In recent times syntheticpolycrystalline diamond films have been successfully produced, such asby chemical vapor deposition (CVD), and used commercially in wearapplications. The synthetic diamond films can be deposited directly onthe base of a wear component, for example as a thin film (generallydefined as a film having a thickness of less than 20 microns), orproduced separately, generally as a thick film, and mounted on the baseof a wear component, such as by brazing.

Even exceedingly hard diamond surfaces have a limited wear life, and thewear life of synthetic diamond, which can vary considerably fordifferent synthetic diamond material, is a key factor in thecost-effectiveness of a wear component. A number of factors arerecognized as affecting the wear life of synthetic diamond. The presenceof foreign matter, voids, and cracks are all usually deleterious to thewear resistance of diamond. In this context, foreign matter alsoincludes carbon not possessing the diamond structure, such as graphite.

It is among the objects of the present invention to provide syntheticdiamond having improved wear properties, an improved method of makingsuch diamond, and a method of determining the suitability of syntheticdiamond for wear applications.

SUMMARY OF THE INVENTION

Applicant has discovered that the wear properties of synthetic diamondare related to the equivalent strain in the diamond crystal structure inan unexpected way, and this discovery is utilized in the invention toproduce diamond with superior wear characteristics, as well as tomonitor and modify processes of diamond deposition to obtain improvedwear surfaces and wear components. As used herein, a wear surface isintended to mean a surface employed for tribological application; forexample, without limitation, a cutting surface, a grinding surface, abearing surface or a valve surface. A wear component is intended to meana wear surface secured to a base element; for example, withoutlimitation, a cutting component, a grinding component, a bearingcomponent, or a valve component. Surprisingly, a certain minimumequivalent strain in the crystal lattice (that is, a displacement ofatom positions with respect to positions they would occupy in a perfectlattice) results in improvement of the wear characteristics of syntheticdiamond, rather than a degrading of wear characteristics that might beexpected from lattice imperfection. In most ceramic applications, lessperfect crystals exhibit poorer wear resistance.

A relatively high thermal conductivity can contribute to improvement ofthe wear characteristics of synthetic diamond. Applicant hasdemonstrated that the obtainment of diamond having substantialequivalent strain improves wear characteristics for synthetic diamondfilm of a given thermal conductivity. Measurements of equivalent straincan also be used in the monitoring and control of synthetic diamond filmproduction and in selection of synthetic diamond for wear applications.

In accordance with a form of the invention, a method is set forth formaking a wear component. A base surface is provided. A synthetic diamondfilm is produced, the film having a thickness of at least 20 microns andan equivalent strain of at least 0.08 percent. The synthetic diamondfilm is applied to the base surface. The film can be deposited directlyon the base surface, for example by having the base surface be thetarget surface in a chemical vapor deposition system. More typically,for thick film synthetic diamond, the film can be separately producedand then mounted on the base surface, such as by brazing a piece ofsynthetic diamond film to a tungsten carbide base surface in a mannerwell known in the art.

It is preferable to form a diamond film wear surface having a thermalconductivity of at least about 9 W/cm°K. For thermal conductivities lessthan about 9 W/cm°K., the diamond film should be formed with anequivalent strain of at least about 0.10 percent to exhibit improvedwear characteristics in accordance with the principles hereof.

In a further form of the invention, there is set forth a method forproducing synthetic diamond film for use as a wear surface. Syntheticdiamond is formed by chemical vapor deposition using initial depositionparameters. The equivalent strain of the diamond film is monitored, forexample by measuring equivalent strain of each sample produced or of asample from a produced batch. The deposition parameters are thenmodified when the equivalent strain of the synthetic diamond is lessthan a predetermined minimum percentage. In an embodiment hereof, theminimum equivalent strain percentage is about 0.08 percent. In a form ofthe invention, the thermal conductivity of the diamond is also measured.In this embodiment, the referenced minimum equivalent strain percentageis about 0.08 percent when the thermal conductivity is greater thanabout 9 W/cm°K., and the referenced minimum equivalent strain percentageis about 0.10 percent when the thermal conductivity is less than about 9W/cm°K. The deposition parameters to be modified include at least oneparameter selected from the group consisting of the ratio of feedstockgases for chemical vapor deposition and the deposition temperature.

In accordance with a further form of the invention, a method is setforth for inspecting synthetic diamond to be utilized as a wear surfaceto select synthetic diamond that is expected to have superior wearproperties. In accordance with this method, synthetic diamond having anequivalent strain of greater than a predetermined percentage is selectedfor use in certain wear application(s), whereas the synthetic diamondthat does not meet the predetermined criteria (as described above) canbe used for applications where the superior wear properties are notrequired, such as in lower cost wear components.

Further features and advantages of the invention will become morereadily apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a plasma jet deposition system 200 of atype which can be utilized in practicing an embodiment of the invention.

FIG. 2 is a top view of a wear component of a type typically utilized asan insert for a milling machine cutting tool.

FIG. 3 is a side view of the wear component of FIG. 2.

FIG. 4 is an operational flow diagram of a procedure in accordance withan embodiment of the invention for producing synthetic diamond havingimproved wear characteristics.

FIG. 5 is an operational flow diagram of a procedure of the inventionfor selecting synthetic diamond based on suitability for wearapplications.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a diagram of a plasma jet depositionsystem 200 of a type which can be utilized in practicing an embodimentof the invention. Reference can also be made to U.S. Pat. Nos. 4,471,003and 4,487,162. The system 200 is contained within a vacuum housing 211and includes an arc-forming section 215 which comprises a cylindricalanode 291, a rod-like cathode 292, and an injector 295 mounted adjacentthe cathode so as to permit injected fluid to pass over the cathode 292.In the illustrated system the input fluid may be a mixture of hydrogenand methane. The anode 291 and cathode 292 are energized by a source ofelectric potential (not shown), for example a DC potential. Cylindricalmagnets, designated by reference numeral 217, are utilized to controlthe plasma generated at the arc forming section. The magnets maintainthe plasma within a narrow column until the plasma reaches thedeposition region 60. Cooling coils 234, in which liquid nitrogen can becirculated, are located within the magnets and surround the focusedplasma.

In operation, a mixture of hydrogen and methane is fed to the injector295, and a plasma is obtained in front of the arc forming section andaccelerated and focused toward the deposition region. The temperatureand pressure at the plasma formation region are typically in theapproximate ranges 1500-2700 degrees C. and 100-700 torr, respectively,and in the deposition region are in the approximate ranges 800-1100degrees C. and 10-200 torr, respectively. As is known in the art,synthetic polycrystalline diamond can be formed from the describedplasma, as the carbon in the methane is selectively deposited asdiamond, and the graphite which forms is dissipated by combination withthe hydrogen facilitating gas.

The bottom portion 105A of the chamber has a base 106 that can mount amedium 62 on which the synthetic diamond is to be deposited. The basecan include a temperature controller. The medium 62 may be, for example,the base of a tool, an insert for a tool, the base of a bearing surface,etc. Alternatively, the medium 62 can be a substrate, such as molybdenumor graphite, on which synthetic diamond can be deposited, removed, andapplied to a base to form a wear surface, such as for a tool or bearingor other wear component.

FIGS. 2 and 3 illustrate an example of a wear component 20 of a typetypically utilized as an insert for a milling machine. A tungstencarbide element or body 24 is provided in a generally rectangular shapewith a chamfered corner having a depression 26 that receives a piece ofdiamond 28 that serves as a cutter. As is known in the art, the diamond28 can be synthetic polycrystalline diamond film. The diamond can bedeposited directly on the element 24 or, more typically for diamondthick films, a piece of synthetic diamond film that is mounted, such asby brazing, on the element 24.

In accordance with the improvement of a form of the present invention,the diamond wear surface comprises a polycrystalline synthetic diamondthick film (i.e. a film at least 20 microns thick) having an equivalentstrain of at least 0.08 percent. For thick film synthetic diamond wearsurfaces having a thermal conductivity greater than about 9 W/cm°K. theequivalent strain should be at least about 0.08 percent to obtainsuperior wear characteristics. When the thermal conductivity is lessthan about 9 W/cm°K., an equivalent strain of at least about 0.10percent is needed to provide the desired superior wear characteristics.The relationship between equivalent strain and wear properties isparticularly significant at substantial thick film thicknesses, viz.greater than about 200 microns.

As is known in the art, certain operating parameters of the plasma jetdeposition equipment, or of other chemical vapor deposition equipments,are generally adjustable, such as by varying the ratio of feedstockgases, (for example, the relative percentages of hydrogen and methane),varying the temperature, and/or varying the pressure of the plasmaand/or the target (e.g. the medium 62 in FIG. 1). In accordance with anaspect of the present invention, operating conditions are adjusted toobtain the desired minimum equivalent strain percentage. Increasing thedeposition temperature tends to increase equivalent strain.

In an embodiment hereof, equivalent strain is measured as follows: X-raydiffraction measurements are made using Cu Kα radiation (45 kV, 40 mA onthe tube) on a Philips PW1700 X-ray diffraction analyzer machine. Thepatterns are recorded for the diamond as grown (i.e. without crushing).The angle 2θ is scanned in 0.04 degree steps, dwelling 1 second at eachstep. Data are corrected in a standard manner using the Philips programprovided for the user of the machine. In particular, a correction usingthe Lorentz polarization factor is made and the Kα2 peaks are strippedassuming the Kα2/Kα1 ratio is 0.5. Instrument broadening is estimatedusing 5-10 μm diamond powder as a standard. Finally, the integralbreadth d of the 331 reflection is measured and the equivalent strain ecomputed from it assuming all of the broadening is induced by strain,using the formula

    d=(e/4)tanθ

where θ is the usual diffraction angle, that is, 70.2 degrees for the331 reflection. See H. P. Klug, L. A. Alexander "X-ray DiffractionProcedures for Polycrystalline and Amorphous Materials", p661,Wiley-Interscience, New York, 1974. The technique of measurement ofequivalent strain is not, of itself, an inventive feature hereof, and itwill be understood that any suitable technique for determiningequivalent strain can be used, consistent with the principles of theinvention.

It is known that an increase in the thermal conductivity can be achievedin CVD diamond by varying the chemistry of the depositing gas. Forexample, additions of oxygen and reductions in carbon content are bothknown methods of increasing thermal conductivity of CVD diamond. Thermalconductivity in an embodiment hereof is measured by a method describedin Enguhard et al, Materials Science & Engineering, Volume B5, pp.127-134 (1990).

FIG. 4 is an operational flow diagram of a procedure in accordance withan embodiment of the invention for producing synthetic diamond havingimproved wear characteristics. The block 410 represents the initializingof deposition operating parameters, for example the operating parametersof the FIG. 1 arc jet plasma deposition equipment. These parameters mayinclude, inter alia, the ratio of feedstock gases, the control oftemperature, and the control of pressure. The block 420 represents thedeposition of synthetic diamond, and the block 430 represents themeasurement of the equivalent strain of the synthetic diamond. Thethermal conductivity of the synthetic diamond can also be measured, asrepresented by the block 440. If the measured equivalent strainpercentage is below a predetermined threshold (which, as noted above,may also take into account the thermal conductivity, a decision is made(represented by decision block 450) to modify the deposition parameters.For example, the deposition temperature can be raised, which tends toincrease the equivalent strain of the resultant CVD diamond. Also, thepercentage of methane in the feedstock gases can be decreased, ifnecessary, to increase the thermal conductivity of the resultant CVDdiamond. When the equivalent strain is above the desired minimum, theprocess can be periodically monitored, as represented by re-entry to theblock 420.

In accordance with a further aspect of the invention, and as representedby the operational flow diagram of FIG. 5, measurement of equivalentstrain is utilized in selecting diamond samples (or selecting batches orproduction runs from which samples are taken) for suitability in wearapplications. Diamond samples (or runs or batches) which do not meetequivalent strain criteria can be used in applications where somecompromise in wear properties may be acceptable, such as in lower costwear components. The block 510 represents the production of thick filmpolycrystalline diamond. The block 520 represents the performance oftesting for equivalent strain on a sample of the producedpolycrystalline synthetic diamond. Thermal conductivity can also bemeasured, as represented by block 530. If the measured equivalent strainpercentage is above a predetermined threshold (which, as noted above,may also take into account the thermal conductivity), the diamond can beaccepted for a particular wear surface application or, conversely,rejected for such application (decision block 540).

EXAMPLES

Equipment of the type shown in FIG. 1, but without cooling coils, wasutilized to produce synthetic diamond samples that were subjected to anumber of wear tests. The tests performed on the samples were asfollows:

Milling Test

The diamond sample, at least 250 um thick, is ground flat and parallelwithin 10 um, and polished on one side. The sample is then mounted withthe finest grains uppermost by brazing on a tungsten carbide insert asillustrated in FIGS. 2 and 3. The insert is mounted as a single tooth ina vertical spindle mill running at 1500 m/min. with an infeed of 0.25mm/rev. and 1 mm depth of cut for a single tooth. The workpiece diameteris 100 mm. Performance is determined by implementing 100 passes of thetool head over a continuous cast A390 aluminum alloy workpiece andmeasuring the wear on the insert using a toolmaker's microscope.

Sandblast Test

The side of the sample that was in contact with the substrate is blastedwith 120 grit SiC flowing at a rate of about 3 g/min. Air pressure isabout 80 psig and the nozzle size is about 0.7 mm. Performance isdetermined by blasting for 25 minutes at a standoff of 2 cm andmeasuring the depth of the pit so formed in um.

EXAMPLE 1

Samples 1-1 and 1-2 were produced using the following conditions of theDC are jet plasma deposition equipment

    ______________________________________                                                      Sample 1-1                                                                              Sample 1-2                                            ______________________________________                                        Gas enthalpy    40.8 kJ/g   40.7 kJ/g                                         % CH4 in H2     0.16        0.16                                              Pressure        20 Torr     20 Torr                                           Deposition temp.                                                                              1050 C.     1050 C.                                           ______________________________________                                    

The samples were measured and tested to determine equivalent strainthermal conductivity, and milling wear, using the measurements and testsfirst described above. The results were as follows:

    ______________________________________                                                     Sample 1-1 Sample 1-2                                            ______________________________________                                        Eq. strain     .13          .08                                               Thermal cond.  5.8 W/cm°K.                                                                         5.3 W/cm°K.                                Milling wear   .0077 in.    .0121 in.                                         ______________________________________                                    

It is seen from this example that two materials made under similarconditions can have substantially differences in equivalent strain. Thematerial with higher equivalent strain exhibited much better performancein the milling test. Specifically, the sample 1-2 had about 57% morewear than the sample 1-1.

EXAMPLE 2

Samples 2-1 and 2-2 were produced using the following conditions of theDC arc jet plasma deposition equipment

    ______________________________________                                                      Sample 2-1                                                                              Sample 2-2                                            ______________________________________                                        Gas enthalpy    178 kJ/g    170 kJ/g                                          % CH4 in H2     0.2         0.35                                              Pressure        4.5 Torr    2.9 Torr                                          Deposition temp.                                                                              900 C.      1050 C.                                           ______________________________________                                    

The samples were measured and tested to determine equivalent strain,thermal conductivity and sandblast test performance, using themeasurements and tests first described above. In this Example (only)thermal conductivity was measured using a method more suitable forsmaller samples, and described in Frederikse et al., Applied Optics,V27, pp. 4672-4675 (1988). The results were as follows:

    ______________________________________                                                     Sample 2-1 Sample 2-2                                            ______________________________________                                        Eq. strain     <0.03        .12                                               Thermal cond.  3.7 W/cm°K.                                                                         4.5 W/cm°K.                                Sandblast pit depth                                                                          85 um        65 um                                             ______________________________________                                    

The sample 2-1, made at substantially lower deposition temperature thansample 2-2 or the samples of Example 1, exhibited much lower equivalentstrain than the other samples. Again, the sample with lower equivalentstrain (2-1) exhibited greater wear; viz., about 30% deeper pit depththan sample 2-2. The lower pressure in making sample 2-2 is believed tohave contributed to this sample having a higher thermal conductivitynotwithstanding the lower percentage of methane used in making sample2-1.

EXAMPLE 3

Samples 3-1 and 3-2 were produced using the following conditions of theDC are jet plasma deposition equipment

    ______________________________________                                                      Sample 3-1                                                                              Sample 3-2                                            ______________________________________                                        Gas enthalpy    46 kJ/g     41 kJ/g                                           % CH4 in H2     0.07        0.17                                              Pressure        20 Torr     10 Torr                                           Deposition temp.                                                                              1050 C.     1050 C.                                           ______________________________________                                    

The samples were measured and tested to determine equivalent strain,thermal conductivity and milling wear, using the measurements and testsfirst described above. The results were as follows:

    ______________________________________                                                    Sample 3-1  Sample 3-2                                            ______________________________________                                        Eq. strain    .099          .093                                              Thermal cond. 11.0 W/cm°K.                                                                         7.0 W/cm°K.                                Milling wear  .0073 in.     .0099 in.                                         ______________________________________                                    

In this example the samples differ mainly in thermal conductivity and,as expected, the material having substantially higher thermalconductivity performed considerably better in the milling wear test.Sample 3-2 had about 35% more wear than sample 3-1.

EXAMPLE 4

Samples 4-1 and 4-2 were produced using the following conditions of theDC are jet plasma deposition equipment

    ______________________________________                                                      Sample 4-1                                                                              Sample 4-2                                            ______________________________________                                        Gas enthalpy    34 kJ/g     35 kJ/g                                           % CH4 in H2     0.081       .088                                              Pressure        10 Torr     10 Torr                                           Deposition temp.                                                                              900         1050 C.                                           ______________________________________                                    

The samples were measured and tested to determine equivalent strain,thermal conductivity and milling wear using the measurements and testsfirst described above. The results were as follows:

    ______________________________________                                                     Sample 4-1 Sample 4-2                                            ______________________________________                                        Eq. strain     .042         .17                                               Thermal cond.  12 W/cm°K.                                                                          8.5 W/cm°K.                                Milling wear   .0113 in     .0085 in                                          ______________________________________                                    

This example shows two materials with thermal conductivities greaterthan 8.5 W/cm K. and differing equivalent strains. The sample with lowerequivalent strain (4-1) exhibited about 33% more milling wear.

In summary, the invention, in its various aspects, has been based uponthe surprising discovery that synthetic diamond films having at least acertain equivalent strain percentage have substantially better wearproperties than seemingly more "perfect" synthetic diamond films havinga lower equivalent strain percentage.

The invention has been described with reference to particular preferredembodiments, but variations within the spirit and scope of the inventionwill occur to those skilled in the art. For example, it will beunderstood that other types of chemical vapor deposition, such asmicrowave plasma deposition or hot filament deposition, can be utilizedto produce synthetic diamond film.

I claim:
 1. A method for producing synthetic diamond for use as a wearsurface, comprising the steps of:depositing synthetic diamond bychemical vapor deposition utilizing initial diamond parameters;monitoring the equivalent strain of the synthetic diamond; modifying atleast one of said deposition parameters when the equivalent strain ofthe synthetic diamond is less than a preselected percentage, so as toeffect a modification of equivalent strain; and depositing furthersynthetic diamond utilizing the modified deposition parameters.
 2. Themethod as defined by claim 1, wherein said preselected equivalent strainpercentage is at least about 0.08 percent.
 3. The method as defined byclaim 2, further comprising monitoring the thermal conductivity of saidsynthetic diamond, and modifying said at least one deposition parameterwhen said thermal conductivity is less than a predetermined magnitude.4. The method as defined by claim 3, wherein said chemical vapordeposition utilizes methane and hydrogen feedstock gases, and whereinsaid deposition parameters comprise at least one parameter selected fromthe group consisting of the ratio of feedstock gases and the depositiontemperature.
 5. The method as defined by claim 2, wherein said at leastone deposition parameter is deposition temperature.
 6. The method asdefined by claim 5, wherein said modifying step comprises increasing thedeposition temperature.
 7. The method as defined by claim 1, furthercomprising monitoring the thermal conductivity of said syntheticdiamond, and modifying said at least one deposition parameter when saidthermal conductivity is less than a predetermined magnitude.
 8. Themethod as defined by claim 7, wherein said chemical vapor depositionutilizes methane and hydrogen feedstock gases, and wherein saiddeposition parameters comprise at least one parameter selected from thegroup consisting of the ratio of feedstock gases and the depositiontemperature.
 9. The method as defined by claim 1, wherein said at leastone deposition parameter is deposition temperature.
 10. The method asdefined by claim 9, wherein said modifying step comprises increasing thedeposition temperature.
 11. The method as defined by claim 1, furthercomprising applying said further synthetic diamond to a wear componentbase to form a wear component.